Regulation of spatio-temporal gene expression in diverse cell and tissue types is a critical aspect of development. Progression through Caenorhabditis elegans vulval development leads to the generation of seven distinct vulval cell types (vulA, vulB1, vulB2, vulC, vulD, vulE, and vulF), each with its own unique gene expression profile. The mechanisms that establish the precise spatial patterning of these mature cell types are largely unknown. Dissection of the gene regulatory networks involved in vulval patterning and differentiation would help us understand how cells generate a spatially defined pattern of cell fates during organogenesis. We disrupted the activity of 508 transcription factors via RNAi and assayed the expression of
ceh-2, a marker for vulB fate during the L4 stage. From this screen, we identified the tailless ortholog
nhr-67 as a novel regulator of gene expression in multiple vulval cell types. We find that one way in which
nhr-67 maintains cell identity is by restricting inappropriate cell fusion events in specific vulval cells, namely vulE and vulF.
nhr-67 exhibits a dynamic expression pattern in the vulval cells and interacts with three other transcriptional regulators
cog-1 (Nkx6.1/6.2),
lin-11 (LIM), and
egl-38 (Pax2/5/8) to generate the composite expression patterns of their downstream targets. We provide evidence that
egl-38 regulates gene expression in vulB1, vulC, vulD, vulE, as well as vulF cells. We demonstrate that the pairwise interactions between these regulatory genes are complex and vary among the seven cell types. We also discovered a striking regulatory circuit that affects a subset of the vulval lineages:
cog-1 and
nhr-67 inhibit both one another and themselves. We postulate that the differential levels and combinatorial patterns of
lin-11,
cog-1, and
nhr-67 expression are a part of a regulatory code for the mature vulval cell types.